Is there any way to use Safari 5.1's rendering engine for browser testing on OSX 10.8 (Mountain Lion)?

I have OSX 10.8.2 (Mountain Lion) on both my Macs, which comes with Safari 6.1. A couple of my clients have been reporting minor CSS errors using Safari 5.1, which I obviously can't install. This makes fixing these errors very, very difficult.

Are there any recommended methods for testing with the rendering engines of older versions of Safari? Would there be an appropriate vintage version of Webkit that would be able to install on 10.8, as the appropriate nightlies refuse to run?

I found a project online which i think uses an old WEBKIT renderer.. http://michelf.ca/projects/multi-safari/ gives old versions, but i don't know which ones you will be able to run on your new machine. For myself, I'm running 4.0.5 because of the reduced RAM usage.